Types of flow meters

1. Positive displacement flowmeter：

A positive displacement flowmeter is equivalent to a standard volume container, which continuously measures the flowing medium. The larger the flow, the more measurements are taken and the output is more frequently. The principle of positive displacement flowmeter is relatively simple, suitable for measuring fluid with high viscosity and low Reynolds number.

According to the shape of the rotator, the current products are divided into: oval gear flowmeter, waist wheel flowmeter (Roots flowmeter), rotary piston and scraper flowmeter suitable for measuring liquid flow; servo flowmeter suitable for measuring gas flow Volumetric flowmeter, membrane type and simple flowmeter, etc.

2. Impeller flowmeter：

The working principle of the impeller flowmeter is that the impeller is placed in the measured fluid, and it is rotated by the impact of the fluid flow, and the speed of the impeller rotation reflects the flow rate. Typical impeller flowmeters are water meters and turbine flowmeters, and their structures can be mechanical transmission output or electric pulse output. Generally, the accuracy of the water meter output by mechanical transmission is low, the error is about±2%, but the structure is simple, the cost is low, the accuracy of the turbine flowmeter output by the electric pulse signal is high, the general error is±0.2%-0. 5%.

3. Differential pressure flowmeter：

The differential pressure flowmeter consists of a primary device and a secondary device. The primary device is called a flow measuring element, which is installed in the pipeline of the measured fluid to generate a pressure difference proportional to the flow (flow rate) for the secondary device to display the flow. The secondary device is called display instrument. It receives the differential pressure signal generated by the measuring element and converts it into the corresponding flow for display. The primary device of the differential pressure flowmeter is usually a throttling device or a dynamic pressure measuring device (pitot tube, uniform velocity tube, etc.). The secondary device is a variety of mechanical, electronic and combined differential pressure gauges with flow display instruments. The differential pressure sensitive elements of the differential pressure gauge are mostly elastic elements. Since the differential pressure and the flow rate are in the square root relationship, the flow display instruments are equipped with square root devices to linearize the flow scale. Most of the instruments are also equipped with a flow integration device to display the accumulated flow for economical calculation.

This method of using differential pressure to measure flow has a long history and is relatively mature. It is generally used in more important occasions all over the world, accounting for about 70% of various flow measurement methods. This meter is used for flow measurement of main steam, feed water, condensate, etc. in power plants.

4. Variable area flowmeter：

The float placed in the conical channel with a large top and a small bottom moves under the force of fluid flowing from bottom to top. When this force is balanced with the "displayed weight" of the float (the weight of the float minus the buoyancy of the fluid it is exposed to), the captive is at rest. The rest height of the float can be used as a measure of the flow rate. Since the cross-sectional area of the flowmeter varies with the height of the float, and the pressure difference between the upper and lower parts of the float is equal when the float is stable, this type of flowmeter is called a variable area flowmeter or an equal pressure drop flowmeter. A typical instrument of this type of flowmeter is a rotor (float) flowmeter.

5. Momentum flowmeter：

A flowmeter that uses the momentum of the measured fluid to reflect the flow rate is called a momentum flowmeter. This type of flowmeter mostly uses detection elements to convert momentum into pressure, displacement or force, etc., and then measure the flow. Typical instruments for such flowmeters are target and rotating vane flowmeters.

6. Impulse flowmeter：

A flowmeter that uses the impulse theorem to measure flow is called an impulse flowmeter, which is mostly used to measure the flow of granular solid media, and is also used to measure the flow of mud, crystalline liquid, and abrasives. The flow measurement range is from a few kilograms per hour to nearly ten thousand tons. A typical instrument is a horizontal component force flowmeter. Its measurement principle is that when the measured medium falls freely from a certain height h onto a detection plate with an inclination angle, an impulse is generated. The horizontal component of the impulse is proportional to the mass flow rate, so Measuring this horizontal component can reflect the size of the mass flow. According to the detection method of signal (9), this type of flowmeter is divided into displacement detection type and direct force measurement type.

7. Electromagnetic flowmeter：

The electromagnetic flowmeter is based on the principle that the electric conductor moves in a magnetic field to generate an induced electromotive force, and the induced electromotive force is proportional to the flow rate, and the principle of reflecting the flow of the pipeline by measuring the electromotive force is made. Its measurement accuracy and sensitivity are high. In industry, it is mostly used to measure the flow rate of water, pulp and other media. The maximum pipe diameter that can be measured is 2m, and the pressure loss is extremely small. However, media with low conductivity, such as gas, steam, etc., cannot be used.

The cost of electromagnetic flowmeter is high, and the signal is easily disturbed by external magnetic field, which affects the wide application in industrial pipe flow measurement. For this reason, the products are continuously improved and updated, and are developing toward microcomputerization.

8. Ultrasonic flowmeter：

Ultrasonic flowmeters are designed based on the principle that the speed of ultrasonic waves propagating in a flowing medium is equal to the geometric sum of the average flow velocity of the measured medium and the speed of the sound wave itself. It also reflects the flow rate by measuring the flow rate. Although the ultrasonic flowmeter only appeared in the 1970s, it is very popular because it can be made into a non-contact type and can be linked with an ultrasonic water level meter to measure the opening flow without disturbing or resisting the fluid. Promising flowmeters.

Ultrasonic Doppler flowmeters manufactured using the Doppler effect have received widespread attention in recent years and are considered ideal instruments for non-contact measurement of two-phase flow.